This invention relates generally to feedthrough capacitor filter assemblies, particularly of the type used in implantable medical devices such as cardiac pacemakers, cardioverter defibrillators and the like, to decouple and shield internal electronic components of the medical device from undesirable electromagnetic interference (EMI) signals. More specifically, this invention relates to an improved feedthrough capacitor filter assembly of the type incorporating a hermetic seal to prevent passage or leakage of fluids through the filter assembly, wherein a laminar flow delamination is provided to accommodate and facilitate post manufacture and pre-usage testing of the hermetic seal.
Feedthrough terminal pin assemblies are generally well known in the art for use in connecting electrical signals through the housing or case of an electronic instrument. For example, in implantable medical devices such as cardiac pacemakers, defibrillators and the like, the terminal pin assembly comprises one or more conductive terminal pins supported by an insulator structure for feedthrough passage of electrical signals from the exterior to the interior of the medical device. Many different insulator structures and related mounting methods are known for use in medical devices wherein the insulator structure provides a hermetic seal to prevent entry of patient body fluids into the medical device housing, where such body fluids could otherwise interfere with the operation of and/or cause damage to internal electronic components of the medical device.
In the past, two primary technologies have been employed to manufacture the hermetic seal. One technique involves the use of an alumina insulator which is sputtered to accept brazing material. This alumina insulator is brazed to the terminal pin or pins, and also to an outer metal ferrule of titanium or the like. The alumina insulator supports the terminal pin or pins in insulated spaced relation from the ferrule which is adapted for suitable mounting within an access opening formed in the housing of the medical device. In an alternative technique, the hermetic seal comprises a glass-based seal forming a compression or fused glass seal for supporting the terminal pin or pins within an outer metal ferrule.
The feedthrough terminal pins are typically connected to one or more lead wires which, in the example of a cardiac pacemaker, sense signals from the patient's heart and also couple electronic pacing pulses from the medical device to the patient's heart. Unfortunately, these lead wires can act as an antenna to collect stray electromagnetic interference (EMI) signals for transmission via the terminal pins into the interior of the medical device. Such unwanted EMI signals can disrupt proper operation of the medical device, resulting in malfunction or failure. For example, it has been documented that stray EMI signals emanating from cellular telephones can inhibit pacemaker operation, resulting in asynchronous pacing, tracking and missed beats. To address this problem, hermetically sealed feedthrough terminal pin assemblies have been designed to include a filter capacitor for decoupling EMI signals in a manner preventing such unwanted signals from entering the housing of the implantable medical device. See, for example, U.S. Pat. Nos. 4,424,551; 5,333,095; 5,751,539; 5,905,627; 5,973,906; 6,008,980; and 6,566,978.
While feedthrough capacitor filter assemblies have provided a significant advance in the art, one potential area of concern is that the filter capacitor is often incorporated into the terminal pin assembly in a way that can mask a defective hermetic seal. More particularly, a defective braze or a defective glass-based seal structure, which would permit undesirable leakage of patient body fluids when mounted on a medical device and implanted into a patient, can be obstructed by the mounting of the filter capacitor and its associated electromechanical connections. For example, with reference to the feedthrough filter capacitor shown in U.S. Pat. No. 4,424,551, a ceramic filter capacitor is bonded to a glass seal and then embedded in epoxy material. Typical post-manufacture leak testing is performed by mounting the feedthrough assembly in a test fixture, and then subjecting one side of the feedthrough assembly to a selected pressurized gas such as helium. While the bulk permeability of the epoxy material is such that helium under pressure can penetrate therethrough in the presence of a defective hermetic seal, the duration of this pressure test (typically a few seconds) is often insufficient to permit such penetration. Accordingly, the epoxy material can mask the defective hermetic seal. The thus-tested feedthrough assembly can then mistakenly be incorporated into a medical device and implanted into a patient, wherein slow leakage of patient body fluids through the feedthrough assembly can cause the medical device to malfunction or fail.
FIGS. 1 and 2, taken from FIGS. 1 and 2 of U.S. Pat. No. 6,566,978, disclose a quadpolar feedthrough capacitor 16 mounted on a quadpolar terminal 10. A gap 38 is formed between the ceramic capacitor 16 and the alumina hermetic seal insulator 36. The purpose of this gap 38 is to allow for ready passage of leak detection gases from the hermetic terminal areas or along lead wire 14 through the insulator 36 to flow to the leak detection vent hole 39. However, providing such a gap 38 between the ceramic capacitor 16 and the insulator 36 surface can result in the tendency to trap contaminants, cleaning solvents or the like into this enclosed space. Conductive polyimides are typically used to form the electrical connection between the lead wire 14 and the inside diameter 22 of the ceramic capacitor 16. Conductive polyimides are also typically used to form the connection between the capacitor 16 outside diameter metallization 25 and the ferrule 26. After curing, these conductive polyimide materials are typically cleaned using a grit blasting system with sodium bicarbonate as the blasting medium. Sodium bicarbonate, otherwise known as baking soda, is highly soluble in water. Accordingly, de-ionized water rinses are used to ensure that no baking soda is left on the part but the sodium bicarbonate dissolves readily into the water cleaning solvent. After drying out, trace elements of the sodium bicarbonate are then left inside any cavity or air gap, for example, the gap 38 formed between the ceramic capacitor 16 and the alumina insulator 36 described in the U.S. Pat. No. 6,566,978. After drying, a sodium bicarbonate residue is very hygroscopic. That is, it will tend to absorb moisture from the surrounding air which can degrade the electrical insulation properties of the feedthrough filtered capacitor assembly 10.
For medical implant applications, it is typical that the insulation resistance requirement be 100 Gigaohms or even higher. In order to consistently achieve an insulation resistance greater than 100 Gigaohms, it is essential that all surfaces be extremely clean. Accordingly, any trace element of sodium bicarbonate or other contaminant left behind leads to rejection of the devices.
Another issue associated with gaps 38 between the ceramic capacitor 16 and the insulator 36 of the ferrule 26 mounting surface is associated with the high voltage requirements of an implantable cardioverter defibrillator. Even low voltage devices like pacemakers are sometimes subjected to high voltage pulses. This is typical during an external defibrillation event. There has been a proliferation of automatic external defibrillators (AEDs) in the marketplace. One can now find AEDs in airplanes, hotels, sporting places and many other public venues. Accordingly, pacemaker wearers are being subjected to an ever-increasing number of high voltage shocks in the patient environment. Referring to FIGS. 2, 6 and 9 of U.S. Pat. No. 6,566,978, one can see that the gap (38, 138 or 238) is an area where electric field enhancement can occur. That is, when a high voltage is applied to the lead wire 18, there could be a tendency for a high electric field stress to occur in the air gap 38. These high electric field stresses can lead to ionization of the air gap 38, a resulting plasma and a catastrophic high voltage breakdown of the device 10. This so called avalanche breakdown would cause an implantable medical device to not function, which would of course be life threatening for a pacemaker or a defibrillator dependent patient.
FIGS. 3 and 4, taken from FIGS. 5 and 6 of U.S. Pat. No. 6,765,779, disclose a unipolar feedthrough capacitor 100 mounted on a unipolar hermetic terminal 102. The feedthrough capacitor 100 incorporates outer diameter metallization 114. An electrical attachment 132 is made from the capacitor outside diameter 114 to the ferrule 118. This connection 132 is typically formed with a high temperature thermal setting conductive polymer such as a conductive polyimide. There are gaps left around the circumference of connection material 132 to provide for helium leak detection pathways. This is generally described in U.S. Pat. No. 6,765,779 in column 2 lines 24 through 67 and in column 3 lines 1 through 33. There is also an axial gap 126 formed between the ceramic feedthrough capacitor 100 and the surface of the hermetic terminal 102. The purpose of this axial gap 126 is so that if there was a defective gold braze 128, 130, helium atoms could readily penetrate the annular space between the lead wire 116 and the inside diameter of the insulator 124. Accordingly, said helium atoms could then pass readily through axial gap 126 and out through the spaces left in the circumferential conductive polyimide attachment material 132.
As previously mentioned, leaving an axial gap 126 can trap contaminants between the capacitor 100 and the insulator 124 or terminal 102 and also has the tendency to enhance (squeeze or compress) electric fields during the application of a high voltage to the device.
Accordingly, there is a need for an EMI filtered hermetic feedthrough terminal assembly suitable for human implant that will avoid the field enhancement issues associated with an air gap, but at the same time provides for a helium leak detection path. The present invention fulfills this need by providing an improved feedthrough capacitor filter assembly suitable for use in an implantable medical device or the like, wherein the feedthrough assembly includes a laminar delamination gap for accommodating and facilitating post-manufactured hermetic seal testing.